He received his Ph.D. in 2007 at the University of Leiden with the thesis on Hittite. In over 1200 pages his dissertation describes the history of Hittite in the light of its Indo-European origin. It consists of two parts. Part One, Towards a Hittite Historical Grammar, contains a description of the Hittite phoneme inventory and a discussion of the sound laws and morphological changes that have taken place between the Proto-Indo-European and the Hittite language stage. Part Two, An Etymological Dictionary of the Hittite Inherited Lexicon, contains etymological treatments of all Hittite words of Indo-European origin. One of the most important conclusions of this dissertation is the confirmation that the Anatolian language group was the first one to split off from Proto-Indo-European and that all other Indo-European branches have undergone a period of common innovations (see Indo-Hittite). The thesis was published in the Leiden-based Indo-European Etymological Dictionary project.
He has written two books:
Etymological Dictionary of the Hittite Inherited Lexicon (= Leiden Indo-European Etymological Dictionary Series 5), 2008, Leiden - Boston, xiii + 1162 p.
Hethitische Texte in Transkription. KBo 35 (= Dresdner Beiträge zur Hethitologie 19), 2006, Wiesbaden, xiv + 353 p (together with D. Groddek)